One of the most critical process control techniques used in the manufacturing of integrated circuits is the measurement of overlay accuracy between successive layers on a wafer (i.e., the determination of how accurately a layer aligns with respect to the layer below it).
Presently, this measurement is done with patterns that are etched into the layers. The relative displacement of the two layers is measured by imaging the patterns at high magnification on an electronic camera and by calculating the image position on the camera using any of a variety of known image analysis algorithms. The most commonly used patterns are concentric squares with dimensions of approximately 20 micrometers on each side, generally referred to as "box within a box" target. This prior art is described and analyzed by Neal T. Sullivan, "Semiconductor Pattern Overlay", in Handbook of Critical Dimensions Metrology and Process Control, pp 160-188, vol. CR52, SPIE Press (1993).
The resulting accuracy of the prior art is therefore limited by the asymmetry of etched line profiles, by aberrations in the illumination and imaging optics, and by image sampling in the camera.
It would be desirable to have a system that overcomes the limitations of the prior art. It is the intent of the present invention to rectify the shortcomings of the prior art in several areas:
1. The present invention illuminates and collects the light over a very narrow field of view, so it does not suffer from off-axis aberrations present in the prior art;
2. The present invention samples the signal at a rate that is as high as necessary to meet the required accuracy (e.g., higher than 1000 samples per micron), while the prior art sampling rate is low (typically less than 10 samples per micron), and limited by the number of pixels in the camera;
3. The alignment requirements for the present invention are loose (on the order of 5 to 10 microns in displacement, and 10 degrees in angle), while the prior art requires high alignment accuracy for repeatable measurements (e.g., 0.1 micron in displacement, 0.01 degrees in angle); and
4. The prior art requires very precise focusing to perform the measurement (about 0.1 micron).
The present invention does not require such focusing accuracy.